1. Field of the Invention
The present invention relates to a method for controlling the output from a consumable electrode arc welding power source. In particular, the present invention relates to a method for removing abnormal voltages superposed on the welding voltage, so that the control of the welding power source is performed based on the normalized welding voltage, thereby stabilizing the state of the welding.
2. Description of the Related Art
In consumable electrode arc welding, it is important to maintain an appropriate value of the apparent arc length (hereinafter simply referred to as arc length), which is the shortest distance between the tip of the consumable electrode (hereinafter simply referred to as welding wire) and the base metal, in order to achieve good welding quality. For this purpose, the consumable electrode arc welding power source performs constant-voltage control. The arc length is detected from the welding voltage based on the proportional relation between the arc length and the welding voltage, and the arc length is maintained at an appropriate value by providing an output control so that the welding voltage becomes equal to a preset voltage value that will give the appropriate arc length. In order to achieve stable control on the arc length, highly accurate detection of the arc length must be made from the welding voltage.
Normally, in consumable electrode arc welding, welding is performed in electrode-positive (EP) polarity where an anode point is formed at a tip of the welding wire while a cathode point is formed at the base metal surface, whereby an arc is formed between the anode point and the cathode point. The anode point formed near the tip of the wire migrates very little. On the contrary, the cathode point migrates back and forth on the base metal surface toward regions including oxidized films. The cathode point also migrates due to contamination on the base metal surface, dynamic state of the molten pool, gas discharged from the molten pool, and so on. A momentary change of the position where the cathode point is formed does not cause the change of the apparent arc length. The apparent arc length changes by a very little amount in a short moment which is not greater than several tens of milliseconds because changes of the apparent arc length are caused by the difference between the wire feeding speed and the wire melting speed. However, the migration of the cathode point which can be caused by many factors as described above introduces an abnormal voltage superimposed on the welding voltage. This abnormal voltage does not have any proportional relation with the apparent arc length. Therefore, if the output control is based on the welding voltage on which the abnormal voltage is superimposed, the abnormal voltage destabilizes the arc length control system, leading to decreased welding quality. Abnormal voltage appears more often in MIG welding and MAG welding in which shield gas is mixed with inert gas (such as argon gas and helium gas). Therefore, the abnormal voltage must be removed from the welding voltage in order to stabilize the arc length control. Hereinafter, conventional art for the removal of the abnormal voltage in consumable electrode pulse arc welding will be described (See JP-A 2003-311409 and JP-A 2005-034853).
FIG. 15 is a chart showing voltage and current waveforms in a consumable electrode pulse arc welding. A time course change of the welding voltage v is shown in (A) whereas a time course change of the welding current i is shown in (B). Explanation will be given below with reference to FIG. 15.
During a predetermined peak rise period Tup from time instant t1 to time instant t2, a transient current which rises from a base current to a peak current flows as shown in (B), and a transient voltage which rises from a base voltage to a peak voltage is applied between the welding wire and the base metal as shown in (A). During a predetermined peak period Tp from time instant t2 to time instant t3, a predetermined peak current flows as shown in (B), and a peak voltage is applied as shown in (A). During a predetermined peak fall period Tdw from time instant t3 to time instant t4, a transient current which falls from the peak current to the base current flows as shown in (B), and a transient voltage which falls from the peak current to the base current is applied as shown (A). During a base period Tb from time instant t4 to time instant t5, a predetermined base current passes flows as shown in (B), whereas a base voltage is applied as shown in (A).
As shown in (A), an abnormal voltage which has a greater value than the normal voltage is superimposed on the base voltage during the base period Tb. In pulse arc welding, control is made by varying the length of the base period Tb so that an average value of the welding voltage v would be equal to a preset voltage value. Therefore, if an abnormal voltage is superimposed on the welding voltage v, error is contained in the arc length detection which is performed on the basis of the welding voltage average value, and the error destabilizes the arc length control.
FIG. 16 illustrates a method of setting a norm voltage waveform for removing the abnormal voltage. First, a norm peak voltage value Vpc, a norm base voltage value Vbc and a fluctuation range ΔVc for a given set of welding conditions such as the kind of welding wire and wire feeding speed are obtained through e.g. experiments. Then, as shown in FIG. 16, the norm voltage waveform is defined by using the following formulas for each lapse of time t, with the start time point of the peak rise period Tup set to be 0.
(11) 0≦t<TupVc=((Vpc−Vbc)/Tup)t+Vbc  Formula (11)(12) Tup≦t<Tup+TpVc=Vpc  Formula (12)(13) Tup+Tp≦t<Tup+Tp+TdwVc=((Vbc−Vpc)/Tdw)(t−Tup−Tp)+Vpc  Formula (13)(14) Tup+Tp+Tdw≦t<Tup+Tp+Tdw+TbVc=Vbc  Formula (14)
For example, here it is assumed that the detected value the welding voltage is vd1 [V] when the time ta is lapsed as shown in the figure. Since the lapse of time ta is within the range defined by Tup+Tp≦ta<Tup+Tp+Tdw, the obtained value is assigned to the above formula (13), which will give a center voltage value Vc1 [V] of the norm voltage waveform expressed as follows:Vc1=((Vbc−Vpc)/Tdw)(ta−Tup−Tp)+Vpc Therefore, the welding voltage detected value vd1 in the lapse of time ta is limited to within a fluctuation range Vc1±ΔVc. Specifically, when vd1≧Vc1+ΔVc, the value is limited to vd1=Vc+ΔVc, whereas when vd1≦Vc1−ΔVc, the value is limited to vd1=Vc−ΔVc. Thus, based on the welding voltage limit value vf calculated in this way, the output control of the welding power source is performed.
FIG. 17 is a waveform chart which shows how an abnormal voltage is removed by using the above-described norm voltage waveform. The waveform of the welding voltage v on which an abnormal voltage is superimposed is shown in (A) as described in FIG. 15, whereas the waveform of the welding voltage limit value vf is shown in (B). In the figure, throughout the period excluding the abnormal voltage period from t1 to t2, the welding voltage v is within the fluctuation range Vc±ΔVc from the norm voltage waveform and therefore, the welding voltage limit value vf=v. On the other hand, during the period from t1 to t2, any welding voltage value v which exceeds the norm voltage waveform fluctuation range upper limit value Vc+ΔVc is limited to the upper limit value, i.e. limited to the welding voltage limit value vf=Vc+ΔV as shown in (B). In this way, it is possible to remove the abnormal voltage included in the welding voltage v, and to extract only the voltage which is proportional to the arc length for use in the output control.
According to the conventional technique described above, it is possible to remove abnormal voltages superimposed on the welding voltage value v in pulse arc welding. However, abnormal voltages superimpose on the welding voltage value v not only in pulse arc welding but also in non-pulse welding such as consumable electrode arc welding. Hereinafter, description will cover this case.
FIG. 18 is a chart showing voltage and current waveforms in consumable electrode arc welding. A time course change of a welding voltage v is shown in (A), whereas a time course change of a welding current I is shown in (B). During a short-circuit period Ts between time points t1-t2, the welding voltage v becomes a short-circuit voltage value of a few volts as shown in (A), and the welding current i increases gradually as shown in (B) During the short-circuit period Ts, the arc is not generated and the cathode point is not formed, and thereby no abnormal voltage is generated. Next, during an arc period Ta between time points t2-t3, the welding voltage v becomes an arc voltage value, and the welding current i decreases gradually as shown in (B). An abnormal voltage occurs during the arc period Ta, as illustrated between time points t4-t5. Since the welding power source is under constant-voltage control, occurrence of an abnormal voltage, i.e. a voltage surge, will cause the welding current i to drop sharply as shown in (B). This change of the current can be a trigger for an unstable arc state. The abnormal voltage is caused by the same factors as described for the case of pulse arc welding.
The abnormal voltage removal method according to the conventional technique for pulse arc welding is not applicable to the abnormal voltage in the above-described consumable electrode arc welding. Here is the reason for this: In pulse arc welding, it is possible to set normal values of the welding voltage for the peak period and for the base period in the form of the norm peak voltage value Vpc and the norm base voltage value Vbc. Therefore, it is possible to define a norm voltage waveform by using these values. In consumable electrode arc welding, however, time course change of the welding voltage v during the arc period Ta varies very diversely depending on the arc load. Because of this, it is not possible, unlike in pulse arc welding described with reference to FIG. 16, to set a norm voltage waveform for each time period. Therefore, it has not been possible to remove abnormal voltages included in the welding voltage with the conventional means in consumable electrode arc welding.